1. Field of the Invention
The present invention relates to a process for the processing of pyrolysis gasoline. More particularly the invention relates to a single stage process for treating the pyrolysis gasoline to remove or convert unwanted contaminants to provide a commercially attractive product.
2. Related Art
Pyrolysis gasoline is a gasoline boiling range petroleum stock obtained as a product or by-product from a process in which thermal processing is used to crack a petroleum stock. One example is the destructive cracking of a naphtha boiling range material to produce ethylene. Another example is the delayed coking of a residual petroleum stock to produce lighter components, including coker gasoline. Products from these thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). The components of the thermal cracking may be any of the various isomers of these compounds. In addition the gasoline boiling range material contains considerable amounts of aromatic compounds and heteroatom compositions such as nitrogen and sulfur containing compounds.
The pyrolysis gasolines are typically processed to removed unwanted acetylenes, diolefins and sulfur compounds. Some of the diolefins may be recovered, especially isoprene. Starting with the product coming from a steam cracker, the valuable C2 C3 and C4 olefins (and in some cases diolefins) are recovered. This leaves a C5+ fraction. Usually the C5 fraction is isolated and hydrogenated in a fixed bed reactor. In some cases isoprene is recovered from this fraction. The remaining C6+ faction is then distilled to isolate a C6-450° F. material suitable for gasoline blending. This fraction, is the pyrolysis gasoline also called “pygas” which must be hydrotreated in order to be blended into gasoline.
Pygas is not stable, and in the prior art treated in a two-stage reactor configuration. The first stage rector is commonly loaded with a Pd or Ni catalyst and operated at moderate temperatures in order to remove very reactive components. Such components include acetylenes, dienes, cyclodienes, styrene and styrenic (alkenyl benzene) compounds. Typically styrene and styrenic levels in the gasoline to the first stage hydrotreater are in the 2 to 8 wt. % range, more typically 2 to 4 wt. %. Sulphur levels are typically in the 100 to 1000 wt. ppm, more typically 100 to 400 ppm. Although the pyrolysis gasoline produced from a first stage hydrotreater is sufficiently stable for gasoline blending, the material often cannot be used because of the sulfur concentration is too high to meet very low sulfur concentration now required in the gasoline pool. To meet sulfur regulations, the product from the first stage is sent to a second stage with CoMo and/or NiMo catalysts to remove S. Following the second stage, it is fairly common that there is further distillation of the pygas to isolate a C6 fraction for benzene extraction, or perhaps even a C7-C9 faction for toluene/xylenes extraction. Thus, it is not important to preserve olefin groups in the second stage, but it is important that aromatics saturation is minimized.
The C5's may be recovered and are useful in isomerization, etherification and alkylation. As noted above, isoprene may also be recovered as a useful product. Normally, however, the diolefins are removed along with acetylenes by selective hydrogenation. The C5's may be completely hydrogenated and returned to a naphtha cracker ethylene plant as recycle.
The C6 and heavier fractions contain sulfur compounds which are usually removed by hydrodesulfurization. The aromatic compounds are often removed and purified by distillation to produce benzene, toluene and xylenes. The aromatic containing fraction is often treated with clay material to remove olefinic material.
Finally the heavy boiling gasoline is normally treated with caustic to remove the mercaptans and olefins prior to being used as a gasoline blending stock. In the present invention many of the separate steps and processes of the prior art are combined into a single multifunctional stage.
A common problem with prior two-stage pygas processes is short run life due to the highly reactive nature of the species in the pygas (even after first stage treatment). Unconverted styrenic compounds and dienes tend to lead to polymer formation and fouling when exposed to the higher temperatures of the second stage. This causes fouling in heaters and high pressure drop across the catalyst bed. It is an advantage of the present invention that a single stage process is provided which avoids fouling and plugging problems, exhibits improved run length in pygas units to increase conversion of styrenics and dienes with nearly full octane retention.